This invention relates to an automatic equalizer for use in equalizing an equalizer input signal supplied through a transmission path or channel with the input signal subjected to a transmission loss which can be classified into a frequency independent and a frequency dependent loss component.
For convenience of expression, the following expressions are used: CMI=Coded Mark Inversion; AMI=Alternate Mark Inversion; WAL1=First Walsh function code, and WAL2=Second Walsh function code.
The transmission path of the type described, is typically a subscriber's communication path between a subscriber's terminal equipment, such as a telephone set, and an exchange. In this event, the equalizer is for use in each of the subscriber's terminal equipment and each input circuit of the exchange, such as each subscriber line circuit. The subscriber's communication path may be a conductor in a cable.
During transmission through such a transmission path, an electrical signal is subjected to a transmission loss. In the manner known in the art, it is possible to classify the transmission loss into two components which will herein be called a first and a second loss component. The first loss component is frequency independent, namely, has no frequency dependency. The second loss component is frequency dependent or has a frequency dependency. Typically, the second loss component increases with the frequency and has a gradient which is linearly proportional to the square root of the frequency.
Various factors either individually or collectively influence the first and the second loss components. The factors are, for example, the kind of the transmission path and the diameter and the length thereof. The first and the second loss components have a mutual relationship which is not unique. An automatic equalizer is therefore desired, which equalizes the equalizer input signal individually for the first and the second loss components, that is, which separately compensates for the first and the second loss components.
Various automatic equalizers of the type are already known. By way of example, an automatic equalizer is described by Toshiro Suzuki et al in a paper submitted to "1983 IEEE International Solid-State Circuits Conference" as Paper No. WPM 7.3 under the title of "A CMOS Switched Capacitor Variable Line Equalizer." The equalizer carries out equalization according to a plurality of compensation characteristics which are determined on the basis of an average loss characteristic of various cables. Another automatic equalizer is reported by Akihiko Takada et al in a paper contributed to "Syowa-59-nendo Densi Tusin Gakkai Soogo Zenkoku Taikai Yoko (1984 National Convention Record of the Institute of Electronics and Electrical Communication Engineers of Japan)" as Paper No. 2314 under the title of "A .sqroot.f Step Equalizer" according to contributors' translation. The latter equalizer comprises a first and a second equalizer which individually compensate for the first and the second loss components and which cooperate in producing an equalized signal as an equalizer output signal. The first and the second equalizers are controlled by a peak voltage of the equalizer output signal in accordance with a distribution law predesigned between those amounts of equalizaton which the first and the second equalizers should carry out. In other words, the first and the second equalizers are subjected to a common control.
Other examples are disclosed in U.S. Pat. No. 4,459,698 issued to Osamu Yumoto et al and in an article which is contributed by Masayuki Ishikawa et al to the IEEE Journal of Solid-State Circuits, Vol. SC-19, No. 4 (August 1984), pages 506.509, and which has a title of "A CMOS Automatic Line Equalizer LSI Chip Using Active-RC Filtering." Like the automatic equalizers according to Takada et al and Suzuki et al, conventional automatic equalizers are disadvantageous in that the first and the second loss components are compensated for only in a predetermined manner and in that an error inevitably remains in the equalized signal to a certain extent.